Changes in water clarity across the shallow continental shelf of the central Great Barrier Reef were investigated from ten years of daily river load, oceanographic and MODIS-Aqua data. Mean photic depth (i.e., the depth of 10% of surface irradiance) was related to river loads after statistical removal of wave and tidal effects. Across the 25,000 km(2) area, photic depth was strongly related to river freshwater and phosphorus loads (R-2 = 0.65 and 0.51, respectively). In the six wetter years, photic depth was reduced by 19.8% and below water quality guidelines for 156 days, compared to 9 days in the drier years. After onset of the seasonal river floods, photic depth was reduced for on average 6-8 months, gradually returning to clearer baseline values. Relationships were strongest inshore and midshelf (similar to 12-80 km from the coast), and weaker near the chronically turbid coast. The data show that reductions in river loads would measurably improve shelf water clarity, with significant ecosystem health benefits. Crown Copyright (C) 2014 Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/3.0/).

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We thank Marites Canto for help in processing the remote sensing data, and the NASA Ocean Biology Processing Group for both the SeaWiFS and MODIS-Aqua satellite-to-in situ matchups for the Secchi depth data. Many thanks to the State of Queensland's Department of Environment and Heritage Protection (DEHP) for providing the wave rider buoy data, the river flow and river nutrient load data, and the sea level observations data, and to the Bureau of Meteorology for providing the rainfall and wind data. Many thanks also to Eric Wolanski for numerous discussions and sharing ideas. The study was funded by the Australian Marine Institute of Marine Science, and the Australian Government's National Environmental Research Program (NERP) Tropical Ecosystems Hub.